Self-made air cleaner noise?

[Phil Thien]

I built an air cleaner with a 1/4-HP three-speed blower on which I got a good deal.

The blower's motor makes a louder hum than I was expecting.

When under power, at any of the speeds, I feel the motor vibrating. As soon as I cut power to the motor, the hum stops immediately. During spin-down, there is just the noise of the moving air.

If I run the thing in high speed and cut the power, the unit coasts to a stop with nothing but the noise of the moving air. So it isn't a balance issue or anything, as the noise is only made under power.

The motor is mounted via vibration isolators. And I'm sure they're helping. But it is still noisy.

Any thoughts? Do your air cleaners made any sort of humming noise? Is this a "feature" of multi-speed motors?

[Thomas Hotchkin]

Phil,
Yes my self made air cleaner is noisy. It's made out of 3/4" birch ply with a single speed 1/3 HP squirrel cage motor assy. When I first started using it, I would use some type of heading protection. First mod was a light dimer switch/motor speed control, very small change in motor speed made very large change in noise/hum. Second mod was to hang box assy. from four hangers made from rubber strips. Air cleaner hangs about two inches below shop ceiling. Now in less I change motor speed I do not hear it running with normal shop back ground noise. ( Radio, hand sanding, hammer, hand saw, and the like). Then again my wife says I don't hear again thing anyway. Selective hearing? Tom

[John M Wilson]

I would suspect that your motor (when under power) is hitting some sort of harmonic frequency with either the box it is in, or the structure it is hanging from.

Harmonics are tricky, since a very small input vibration is amplified to an output all out of proportion with the input. So, a motor (or other vibration source) that appears to be running very smoothly creates an unexpected ruckus.

The (possibly only) good thing about harmonics is that if you can get even a small distance away from the exciting frequency (as Thomas did with his dimmer switch) you can often eliminate any vibration from wherever you are getting the harmonic noise from.

So, if it is a harmonic issue, you have two choices: change the frequency of the input (vary the motor speed) or change the frequency of the resonator (change the shape of the box, change the mounting to the ceiling, etc.)

In a previous life, I worked with the guys at the car factory that would chase down buzzes, squeaks, & rattles in the new cars. It's more art than science.

Good luck!

[glenn bradley]

You can check for cabinet resonance by clamping the cabinet. Without sophisticated equipment and we're just trying to see if the problem is cabinet resonance; one random placement is as good as another. If the noise changes, try another position and another till you get your best result (assuming this works). If you are able to find "sweet spots" that drop the noise level when reinforced, experiment with ways to make those spots more rigid; glue and screw a piece of scrap to the spot maybe(?). My shop made unit noise varies with filter density (resistance?). I found some filters that give me adequate filtering with reduced noise and bought a bunch. Good luck.

[Phil Thien]

Thomas, thanks for the real-world feedback. You've given me some things to think about.

John, I agree that the box is amplifying the sound. I made a mistake in going with a box that is roughly a cube, as I have four surfaces all the same size and all sympathetic to the same frequency. I can always use bracing to break the panels into difference sizes.

I'm just surprised that I can put my hand on the motor and feel as much vibration as I do.

[Steve DiMasi]

Just a thought: have you tried running it without a filter? That might help determine if it's related to that particular brand of filter or if it is a box stiffness issue.

[Phil Thien]

Just a thought: have you tried running it without a filter? That might help determine if it's related to that particular brand of filter or if it is a box stiffness issue.

Yep, with and without. All three speeds.

[Steve DiMasi]

It seems strange that it would have harmonics at all three speeds, with and without the filter. Maybe the squirrel cage is out of balance? I wonder if you could spin it, let it stop, mark the bottom of the cage and then see if it continually stops near the same place. If it does, you could add something for weight to the top.

[Phil Thien]

It seems strange that it would have harmonics at all three speeds, with and without the filter. Maybe the squirrel cage is out of balance? I wonder if you could spin it, let it stop, mark the bottom of the cage and then see if it continually stops near the same place. If it does, you could add something for weight to the top.

As soon as I cut power, the noise stops. It is ONLY when the unit is under power that it is noisy.

When I do cut power (including at the highest speed) it takes a significant time to spin down, and it never makes the noise on the way down.

If I had a way to spin the unit at-speed without another motor that was perfectly silent, the assembly would only make the noise of rushing air.

I suspect this particular motor I've got was designed in such a way that it hums a bit. Or I got a bad motor.

[Steve DiMasi]

Good point. I guess I didn't read your original post very well!

I'm stumped.

[Steve DiMasi]

One more thought - would mounting the motor with rubber isolators or foam help? The motor may still vibrate, but it might not transfer to the cabinet as much.

[Phil Thien]

One more thought - would mounting the motor with rubber isolators or foam help? The motor may still vibrate, but it might not transfer to the cabinet as much.

It came with real nice vibration mounts.

[David Cefai]

Does the hum sound like a 60Hz one. In that case it may be rapid (60Hz) expansion/contraction - either thermal or magnetic, or a component (winding, transformer...).

Like the hum an electric heater emanates, that's thermal, or a transformer that's magnetic + possibly thermal.

[Phil Thien]

Does the hum sound like a 60Hz one. In that case it may be rapid (60Hz) expansion/contraction - either thermal or magnetic, or a component (winding, transformer...).

Like the hum an electric heater emanates, that's thermal, or a transformer that's magnetic + possibly thermal.

If I had to guess, I'd say it was higher than 60-Hz.

I should have used isolation mounts between the blower and the enclosure.

[Greg Peterson]

Have you run the motor without the squirrel connected?

I had a Delta AP-100 that vibrated so bad a fluorescent tube fell out of a nearby fixture. I fashioned a mounting system that eliminated that problem. I have since replaced the Delta with a Jet unit, but I use the same mounting system. See below.

I suppose I could take it up a notch and use some rubber bushings, but frankly that would be addressing a non-existent problem for this unit.

You might also try placing something in each corner of your enclosure to eliminate or minimize standing waves. Or some kind of material that dampens (disrupts) sound waves.